Intercooler

ABSTRACT

An inner fin arranged in a flat tube has a wave shape constructed by alternately connecting first walls and second walls. The first wall connects two of the second walls in a connecting direction. The first wall has a protrusion with an extending dimension in the connecting direction and a protruding dimension protruding from the first wall. A ratio of the extending dimension to a height dimension of the first wall is defined as a length ratio x/Fh, and a ratio of the protruding dimension to a width dimension of the second wall is defined as a protrusion ratio y/Fw. The ratios x/Fh, y/Fw are set to have values having a predetermined relationship.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2010-168478 filed on Jul. 27, 2010, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an intercooler.

2. Description of Related Art

JP-A-2006-90305 (US 2006/0042607 A1) describes an intercooler having a tube and an inner fin arranged in the tube. The inner fin has a wavy cross-section, and the wavy cross-section of the inner fin partitions inside of the tube into passages. The inner fin linearly extends in a flowing direction of intake air, so that the inner fin is called as a straight fin.

The inner fin is constructed by alternately connecting first walls and second walls. The first wall partitions the inside of the tube into the passages, and a face of the second wall is fixed to an inner face of the tube. Both of the first wall and the second wall are constructed by simple planes.

The straight fin has a comparatively small flow resistance when intake air flows through the passages, so that a pressure loss of the intercooler is low. However, a boundary layer of intake air flow is easily generated on each face of the first wall and the second wall. In this case, a heat radiating property of the intercooler may be lowered.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems, it is an object of the present invention to provide an intercooler.

According to a first example of the present invention, an intercooler includes a flat tube and an inner fin arranged inside of the flat tube. While intake air to be drawn into an engine passes through the flat tube, the intake air is cooled by external fluid. The flat tube has two major faces opposing with each other in a thickness direction. The inner fin has a wave-shaped cross-section constructed by alternately connecting first walls and second walls in a major direction approximately perpendicular to the thickness direction. The second wall is approximately parallel with the two major faces. The first wall connects two of the second walls in a connecting direction corresponding to the thickness direction. The first wall linearly extends in a flowing direction of the intake air that is approximately perpendicular to the connecting direction and the major direction. The first wall has a protrusion protruding in the major direction and the protrusion is located at a middle position in the connecting direction. The protrusion is defined to have an extending dimension (x) in the connecting direction, and a protruding dimension (y) protruding from a face of the first wall in the major direction. The first wall is defined to have a height dimension (Fh) in the connecting direction, and the second wall is defined to have a width dimension (Fw) in the major direction. A ratio of the extending dimension to the height dimension is defined as a length ratio (x/Fh), and a ratio of the protruding dimension to the width dimension is defined as a protrusion ratio (y/Fw). When the length ratio (x/Fh) is applied to a lateral axis of a two-axis coordinate, and when the protrusion ratio (y/Fw) is applied to a vertical axis of the two-axis coordinate, the length ratio (x/Fh) and the protrusion ratio (y/Fw) are set to have values in an area surrounded by the vertical axis and lines connecting a point (x/Fh, y/Fw=0, 0), a point (x/Fh, y/Fw=0.89, 0.05), a point (x/Fh, y/Fw=1.0, 0.1), a point (x/Fh, y/Fw=0.87, 0.15), a point (x/Fh, y/Fw=0.77, 0.2), a point (x/Fh, y/Fw=0.64, 0.25), and a point (x/Fh, y/Fw=0, 0.4) in this order.

Accordingly, heat radiating property of the intercooler can be raised.

According to a second example of the present invention, an intercooler includes a flat tube and an inner fin arranged inside of the flat tube. While intake air to be drawn into an engine passes through the flat tube, the intake air is cooled by external fluid. The flat tube has two major faces opposing with each other in a thickness direction. The inner fin has a wave-shaped cross-section constructed by alternately connecting first walls and second walls in a major direction approximately perpendicular to the thickness direction. The second wall is approximately parallel with the two major faces. The first wall connects two of the second walls in a connecting direction corresponding to the thickness direction. The first wall linearly extends in a flowing direction of the intake air that is approximately perpendicular to the connecting direction and the major direction. The second wall has a protrusion protruding from an inner face of the second wall inward in the connecting direction.

Accordingly, heat radiating property of the intercooler can be raised.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

FIG. 1 is a schematic front view illustrating an intercooler according to a first embodiment;

FIG. 2 is a schematic cross-sectional view taken along line II-II of FIG. 1;

FIG. 3 is a perspective view illustrating a protrusion of an inner fin of the intercooler;

FIG. 4A is a front view illustrating the protrusion, and FIG. 4B is a side view illustrating the protrusion;

FIG. 5 is a side view illustrating a height dimension of the intercooler and an extending dimension of the protrusion;

FIG. 6 is a front view illustrating a width dimension of the intercooler and a protruding dimension of the protrusion;

FIG. 7 is a graph illustrating a relationship between a length ratio and a density ratio of supercharged air;

FIG. 8 is a graph illustrating a relationship between the length ratio and a protrusion ratio;

FIG. 9A is a simulation model illustrating a flowing velocity distribution of intake air in a tube of the intercooler, and FIG. 9B is a simulation model illustrating a flowing velocity distribution of intake air in a tube of an intercooler of a comparison example;

FIGS. 10A-10D are views respectively illustrating modifications of the protrusion according to a second embodiment;

FIG. 11 is a schematic perspective view illustrating a protrusion of an inner fin of an intercooler according to a third embodiment; and

FIG. 12 is a front view illustrating an intercooler according to other embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT First Embodiment

A first embodiment will be described with reference to FIGS. 1-9B. As shown in FIG. 1, an air-cooled type intercooler 100A has a few number of tubes 111, and the tubes 111 are comparatively long.

Intake air is compressed by a turbocharger (not shown), and the compressed air is drawn into an engine (not shown) of a vehicle. The intake air may be hereinafter referred as supercharged air. The intercooler 100A is a heat exchanger to cool the intake air by exchanging heat with cool air corresponding to external fluid. The intercooler 100A mainly has a core part 110 and a pair of header tanks 120, 130. Each component of the intercooler 100A is made of aluminum or aluminum alloy which is excellent in thermal conductivity. The intercooler 100A is produced by brazing, welding or swaging its components.

The core part 110 is constructed by alternately layering the tubes 111 and outer fins 112. An inner fin 114 is arranged in the tube 111. A side plate 113 is arranged on the outer fin 112 located most outside.

Intake air passes through the tube 111, and the tube 111 has a flat rectangular cross-section, as shown in FIG. 2. A cross-sectional area of the tube 111 is made large as much as possible within a limited space, so as to reduce a flow resistance of intake air.

The flat tube 111 has two major faces 111 a and two minor faces 111 b. The major face 111 a is approximately parallel with a major side of the flat cross-section, and the minor face 111 b is approximately parallel with a minor side of the flat cross-section. An inner face of the tube 111 is defined as a tube inner face 111 c. The major side of the flat cross-section is defined to extend in a major direction.

The outer fin 112 is produced to have a wave shape by processing a thin plate material. Plural louvers 112 a are defined in a plane part of the outer fin 112 by cutting and bending. The outer fin 112 increases an area of radiating (exchanging) heat toward cool air. Further, turbulent effects are generated by the louvers 112 a so as to promote the heat exchange with intake air. A dimension of the outer fin 112 in a flowing direction of cool air is set approximately equal with that of the tube 111.

The side plate 113 is a strengthening member extending in a longitudinal direction of the tube 111. The side plate 113 has an approximately U-shape cross-section, and is arranged on the outer fin 112 located most outside in a tube layering direction. Open side of the U-shape cross-section of the side plate 113 is located outside and opposite from the tube 111 and the outer fin 112.

A mountain (top) part of the outer fin 112 having the wave shape is contact and connected with the major face 111 a of the tube 111. The outer fin 112 located most outside is contact and connected with the side plate 113.

As shown in FIG. 1, the header tank 120, 130 is disposed at an end of the tube 111 in the tube longitudinal direction. The tank 120, 130 extends in the tube layering direction, and communicates with each of the tubes 111. The header tank 120 has a header plate 121, a tank part 122, and an inlet pipe 123. The header tank 130 has a header plate 131, a tank part 132, and an outlet pipe 133.

The header plate 121, 131 has a burring around an outer periphery, and has a tube hole at a position corresponding to the tube 111. The burring has plural swaging nails, and the tank part 122, 132 is mechanically connected to the burring by swaging the nails. The end of the tube 111 is inserted and fitted with the tube hole. The tube 111 and the header plate 121, 131 are contact and connected with each other. An end of the side plate 113 in a longitudinal direction is contact and connected to the header plate 121, and the other end of the side plate 113 is contact and connected to the header plate 131.

The tank part 122, 132 is a semi-container open to the header plate 121, 131. The open side of the tank part 122, 132 is located on an inner side of the burring of the header plate 121, 131. A seal member (not shown) is interposed between the header plate 121, 131 and the tank part 122, 132. The tank part 122, 132 is connected to the header plate 121, 131 by swaging the nails of the header plate 121, 131.

The pipe 123, 133 is a communication portion that makes an inside of the tank part 122, 132 to communicate with outside. The pipe 123, 133 is integrated with the tank part 122, 132. Intake air flows into the tank part 122 through the inlet pipe 123, and is discharged out of the tank part 132 through the outlet pipe 133.

The inner fin 114 is disposed inside the tube 111. The inner fin 114 increases an area of exchanging heat with intake air flowing through the tube 111, so as to promote heat exchange. The inner fin 114 is produced to have a waveform by processing a thin plate material. Because the tube 111 has the flat rectangular cross-section, the inner fin 114 is efficiently arranged in the tube 111 without creating a dead space.

As shown in FIG. 2, the inner fin 114 has a first wall 114 a and a second wall 114 b. The first wall 114 a connects the second walls 114 b in a connecting direction corresponding to an up-and-down direction of FIG. 2. The major faces 111 a of the tube 111 oppose to each other in the connecting direction, and the first wall 114 a extends in the connecting direction. The second wall 114 b is approximately parallel with the major face 111 a of the tube 111, and extends in the major direction corresponding to a left-and-right direction of FIG. 2.

As shown in FIG. 3, an end of the second wall 114 b is connected to the first wall 114 a, and the other end of the second wall 114 b is connected to another first wall 114 a. The inner fin 114 has the waveform by alternately connecting the first walls 114 a and the second walls 114 b in the major direction. The first wall 114 a extends approximately perpendicularly to the major face 111 a in a manner that the connecting direction corresponds to a thickness direction of the flat tube 111. In this case, the waveform of the inner fin 114 is rectangle or square. Alternatively, the connecting direction may be inclined with respect to the thickness direction. In this case, the waveform of the inner fin 114 is trapezoid.

The inner fin 114 is so-called straight type fin. The first wall 114 a linearly extends in a flowing direction of intake air represented by a blank arrow direction in FIGS. 3 and 4B. The first wall 114 a is arranged in the tube 111 so as to connect the major faces 111 a opposing with each other, so that an inside of the tube 111 is divided into plural passages.

The second wall 114 b linearly extends in the flowing direction of intake air, similarly to the first wall face 114 a. A face of the second wall 114 b is contact and connected to the tube inner face 111 c. A width dimension of the first wall 114 a in the connecting direction is set longer than a width dimension of the second wall 114 b in the major direction, and each passage is longer in the connecting direction than in the major direction.

As shown in FIG. 3, the first wall 114 a has a protrusion 114 c, 114 d at a middle position in the connecting direction. For example, the protrusion 114 c, 114 d is located at. a central position in the connecting direction. The protrusion 114 c protrudes leftward in the major direction from the first wall 114 a in FIG. 3, and the protrusion 114 d protrudes rightward in the major direction from the first wall 114 a in FIG. 3. Inside of the protrusion 114 c, 114 d is recessed in the same direction. When the first wall 114 a is seen from front, the protrusion 114 c, 114 d extends in the connecting direction. Specifically, the protrusion 114 c, 114 d has an ellipse shape, as shown in FIG. 4B and 5.

The protrusions 114 c and the protrusions 114 d are alternately arranged in the flowing direction of intake air, on the single first wall 114. When the first walls 114 a oppose to each other in the major direction, positions of the protrusions 114 c correspond with each other in the major direction, and positions of the protrusions 114 d correspond with each other in the major direction.

As shown in FIG. 5, the first wall 114 a of the inner fin 114 is defined to have a height dimension Fh in the connecting direction, and the protrusion 114 c, 114 d is defined to have an extending dimension x in the connecting direction. As shown in FIG. 6, the second wall 114 b is defined to have a width dimension Fw in the major direction, and the protrusion 114 c, 114 d is defined to have a protruding dimension y protruding from the first wall 114 a in the major direction.

A length ratio x/Fh is defined as a ratio of the extending dimension x to the height dimension Fh. A protrusion ratio y/Fw is defined as a ratio of the protruding dimension y to the width dimension Fw. The length ratio x/Fh and the protrusion ratio y/Fw are set to have values within a hatched area of FIG. 8. When the inner fin 114 is defined to have a fin pitch Fp between mountain parts of the wave shape located adjacent to each other, the width dimension Fw is equal to ½ of the fin pitch Fp. A temperature of intake air is raised when the intake air is compressed by a turbocharger (not shown), and the compressed air flows into the tank part 122 through the inlet pipe 123. Intake air is distributed into the tubes 111 from the tank part 122. While intake air flows inside of the tube 111, intake air is cooled by external cool air through heat exchange. That is, heat of intake air is emitted to the external cool air through the inner fin 114, the face 111 a, 111 b of the tube 111, and the outer fin 112. The cooled air is gathered in the tank part 132, and flows out of the outlet pipe 133 so as to be supplied to the engine.

The air-cooled type intercooler 100A has a few number of the tubes 111, and the tubes 111 are comparatively long. Therefore, if intake air of the intercooler 100A is defined to have a pressure loss ΔPg represented by a following Expression 1, the pressure loss ΔPg becomes comparatively large.

ΔPg=4·f·(H/de)·(ρ/2g)·Vg²   (Expression 1)

f=coefficient of friction

H=longitudinal length of the tube

de=diameter of a circle corresponding to the tube

p=density of the supercharged air

g=gravitational acceleration

Vg=flowing velocity of intake air in the tube

The straight type inner fin 114 is arranged in the tube 111 in a manner that the flow resistance of intake air becomes comparatively small. FIG. 9A illustrates a distribution of flowing velocity of intake air in the tube 111 of the first embodiment, and FIG. 9B shows a comparison example. The flowing velocity becomes slower in order of a flowing velocity FR1, a flowing velocity FR2, and a flowing velocity FR3, in FIG. 9A. The flowing velocity becomes slower in order of a flowing velocity FR11, a flowing velocity FR12, a flowing velocity FR13, a flowing velocity FR14, and a flowing velocity FR 15, in FIG. 9B.

As shown in FIG. 9B representing the comparison example, a passage is defined by a wall not having the protrusion 114 c, 114 d, and a boundary layer of intake air flow is easily generated on the second wall 114 b and the tube inner face 111 c corresponding to an opening part of the inner fin 114. The boundary layer causes a decrease in the heat radiating property.

In contrast, according to the first embodiment, the flow resistance of intake air can be maintained low. Further, the protrusion 114 c, 114 d is defined on the first wall 114 a of the inner fin 114, and the length ratio x/Fh and the protrusion ratio y/Fw are set to have values within the hatched area of FIG. 8. Therefore, as shown in FIG. 4B, intake air flowing through the tube 111 can be deflected toward the second wall 114 b and the tube inner face 111 c opposite from with each other. The boundary layer formed on the second wall 114 b and the tube inner face 111 c can be disturbed, so that a thickness of the boundary layer can be reduced. Heat transmitting efficiency can be improved on the intake air side, and the heat radiating property can be raised.

As shown in FIG. 9A in contrast to FIG. 9B, a distribution line of flowing velocity FR1, FR2, FR3 is varied in a direction approaching the second wall 114 b and the tube inner face 111 c. It is confirmed that the thickness of the boundary layer is reduced.

A reason will be described below why the length ratio x/Fh and the protrusion ratio y/Fw are set to have the values within a predetermined range so as to obtain the above advantages. As shown in FIG. 7, an optimal condition to improve a density ratio (ρ/ρ0) of supercharged air is acquired when the length ratio x/Fh is variously changed between 0-1, using the protrusion ratio y/Fw as a. parameter.

The density ratio (ρ/ρ0) of supercharged air is a ratio of a density (ρ) of supercharged air of the first embodiment to a density (ρ0) of supercharged air of the comparison example. The density (ρ) of supercharged air indicates a density of air flowing out of the intercooler 100A, and is represented by a heat radiation performance and a pressure loss of the intercooler 100A. The density (ρ) of supercharged air is computed by the following Expression 2.

ρ=(Pg1-ΔPg)/{R·(Tg1-Qg/Gg·Cp)}  (Expression 2)

Pg1=inlet-side pressure of intake air

ΔPg=pressure loss of intake air of the intercooler

R=gas constant

Tg1=inlet-side temperature of intake air

Qg=heat radiating amount

Gg=Mass flow rate of intake air

Cp=Specific heat of intake air

As the density (ρ) of supercharged air is raised, the pressure loss is reduced and the heat radiation property is made better, in the intercooler 100A. Further, if the density ratio (ρ/ρ0) of supercharged air becomes equal to or higher than 100%, the properties of the intercooler 100A are better than those of an intercooler of the comparison example.

In FIG. 7, when the length ratio x/Fh is increased from 0 to 1.0 with a parameter of the protrusion ratio y/Fw, the density ratio ρ/ρ0 of supercharged air becomes equal to or higher than 100%, and has a maximum value.

Specifically, the density ratio of ρ/ρ0 of supercharged air becomes equal to or higher than 100% and has the maximum value when the protrusion ratio y/Fw has a value of 0.05 and the length ratio x/Fh is in a range of 0-0.89.

The density ratio ρ/ρ0 of supercharged air becomes equal to or higher than 100% and has the maximum value when the protrusion ratio y/Fw has a value of 0.1 and when the length ratio x/Fh is in a range of 0-1.0.

The density ratio ρ/ρ0 of supercharged air becomes equal to or higher than 100% and has the maximum value when the protrusion ratio y/Fw has a value of 0.15 and when the length ratio x/Fh is in a range of 0-0.87.

The density ratio ρ/ρ0 of supercharged air becomes equal to or higher than 100% and has the maximum value when the protrusion ratio y/Fw has a value of 0.2 and when the length ratio x/Fh is in a range of 0-0.77.

The density ratio ρ/ρ0 of supercharged air becomes equal to or higher than 100% and has the maximum value when the protrusion ratio y/Fw has a value of 0.25 and when the length ratio x/Fh is in a range of 0-0.64.

That is, with respect to each protrusion ratio y/Fw, if the length ratio x/Fh is set in the above-mentioned predetermined range, the density p of supercharged air can be raised compared with the comparison example. The pressure loss is reduced, and the heat radiation property of the intercooler 100A can be raised.

As shown in FIG. 8, when the length ratio x/Fh is applied to a lateral axis of a two-axis coordinate, and when the protrusion ratio y/Fw is applied to a vertical axis of the two-axis coordinate, the length ratio x/Fh and the protrusion ratio y/Fw are set to have values in an area surrounded by the vertical axis and lines connecting a point (x/Fh, y/Fw=0, 0), a point (x/Fh, y/Fw=0.89, 0.05), a point (x/Fh, y/Fw=1.0, 0.1), a point (x/Fh, y/Fw=0.87, 0.15), a point (x/Fh, y/Fw=0.77, 0.2), a point (x/Fh, y/Fw=0.64, 0.25), and a point (x/Fh, y/Fw=0, 0.4) in this order. In this case, the combination of the protrusion ratio y/Fw and the length ratio x/Fh causes the density ratio ρ/ρ0 of supercharged air to become more than or equal to 100%. The maximum side value of the length ratio x/Fh of FIG. 7 to make the density ratio 100% or more is set as an upper limit of the length ratio x/Fh with respect to each point.

Especially, in FIG. 7, when the protrusion ratio y/Fw is set as 0.1, the density ratio ρ/ρ0 of supercharged air becomes the largest. At this time, the length ratio x/Fh may be preferably set in a range between 0.43 and 0.87.

The protrusion 114 c, 114 d protruding on a first side in the major direction is produced by pressing the first wall 114 a from a second side toward the first side.

Therefore, the protrusion 114 c, 114 d can be easily formed by a roller processing or a pressing processing when the inner fin 114 is produced.

Second Embodiment

As shown in FIGS. 10A-10D, an inner fin 114 has a protrusion 114 e, 114 f. Shape, number and location of the protrusion 114 e, 114 f are different from those of the protrusion 114 c, 114 d of the first embodiment.

As shown in FIG. 10A, the protrusion 114 e has a circle shape while the protrusion 114 c, 114 d has the ellipse shape. The protrusion 114 e may be formed by dimpling. As shown in FIG. 10B, a plurality of the protrusions 114 e may be arranged in the connecting direction.

As shown in FIG. 10C, the protrusion 114 f has a triangle shape while the protrusion 114 c, 114 d has the ellipse shape. A first angle of the protrusion 114 f having the triangle shape is located on an upstream side in the flowing direction of intake air A second angle and a third angle are located on a downstream side in the flowing direction of intake air, and arranged in the connecting direction. Intake air is effectively deflected toward the second wall 114 b and the tube inner face 111 c opposing with each other in the tube thickness direction.

As shown in FIG. 10D, three of the circle protrusions 114 e are arranged to define an imaginary triangle, and the protrusions 114 e are respectively located at three angle portions of the imaginary triangle. Locations of the angle portions with respect to the flowing direction of intake air are the same as FIG. 10C.

According to the second embodiment, similar advantages can be obtained as the first embodiment, if the length ratio x/Fh and the protrusion ratio y/Fw are set to have values within the hatched area of FIG. 8. In the case of FIGS. 10B and 10D, the extending dimension x of the protrusions 114 e is defined to be entire length of the protrusions 114 e in the connecting direction.

Third Embodiment

As shown in FIG. 11, an inner fin 114 has a protrusion 114 g. A location of the protrusion 114 g is different from that of the protrusion 114 c, 114 d of the first embodiment.

The protrusion 114 g is defined in the second wall 114 b by being pressed from outside to be connected to the tube inner face 111 c. The protrusion 114 g protrudes toward a center side of the tube 111. That is, the protrusion 114 g protrudes from an inner face of the second wall 114 b toward an open side of the inner fin 114 having the waveform. For example, the protrusion 114 g has a circle shape.

According to the third embodiment, the flow resistance of intake air can be maintained low. Further, the protrusion 114 g is defined on the second wall 114 b of the inner fin 114, so that intake air flowing through the tube 111 adjacent to the second wall 114 b can be disturbed by the protrusion 114 g. The boundary layer formed on the second wall 114 b can be disturbed, so that a thickness of the boundary layer can be reduced. Heat transmitting efficiency can be improved on the intake air side, and the heat radiating property can be raised.

Other Embodiment

The above embodiments may be applied to an intercooler 100B shown in FIG. 12, which has a comparatively large number of tubes 111, and the tubes 111 are comparatively short, compared with the intercooler 100A.

The protrusion 114 c, 114 d, 114 e, 114 f, 114 g is not limited to protrude on a first side in the major direction or the connecting direction by pressing the first wall 114 a or the second wall 114 b from a second side opposite from the first side in the major direction or the connecting direction. Alternatively, the protrusion 114 c, 114 d, 114 e, 114 f, 114 g may be formed by cutting and bending the wall 114 a, 114 b. In this case, the cut and separated part is located on a downstream side in the flowing direction of intake air, and the bent and connected part is located on an upstream side in the flowing direction of intake air.

Each component of the intercooler 100A, 100B is not limited to be made of aluminum or aluminum alloy, but may be made of copper-base material or iron base material. The header tank 120, 130 is not limited to be made of aluminum-base, copper-base or iron-base material, but may be made of resin material.

The intercooler 100A, 100B is not limited to the air-cooled type one using air as external fluid to cool the intake air, but may be a water-cooled type one using cooling water as the external fluid.

Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims. 

1. An intercooler comprising: a flat tube, intake air to be drawn into an engine passing through the flat tube and being cooled by external fluid, the flat tube having two major faces opposing with each other in a thickness direction; and an inner fin arranged inside of the flat tube, the inner fin having a wave-shaped cross-section constructed by alternately connecting a plurality of first walls and a plurality of second walls in a major direction approximately perpendicular to the thickness direction, the second wall being approximately parallel with the two major faces, the first wall connecting two of the second walls in a connecting direction corresponding to the thickness direction, wherein the first wall linearly extends in a flowing direction of the intake air that is approximately perpendicular to the connecting direction and the major direction, the first wall has a protrusion protruding in the major direction and being located at a middle position in the connecting direction, the protrusion is defined to have an extending dimension (x) in the connecting direction, and a protruding dimension (y) protruding from a face of the first wall in the major direction, the first wall is defined to have a height dimension (Fh) in the connecting direction, and the second wall is defined to have a width dimension (Fw) in the major direction, a ratio of the extending dimension to the height dimension is defined as a length ratio (x/Fh), and a ratio of the protruding dimension to the width dimension is defined as a protrusion ratio (y/Fw), the length ratio (x/Fh) is applied to a lateral axis of a two-axis coordinate, and the protrusion ratio (y/Fw) is applied to a vertical axis of the two-axis coordinate, and the length ratio (x/Fh) and the protrusion ratio (y/Fw) are set to have values in an area surrounded by the vertical axis and lines connecting a point (x/Fh, y/Fw=0, 0), a point (x/Fh, y/Fw=0.89, 0.05), a point (x/Fh, y/Fw=1.0, 0.1), a point (x/Fh, y/Fw=0.87, 0.15), a point (x/Fh, y/Fw=0.77, 0.2), a point (x/Fh, y/Fw=0.64, 0.25), and a point (x/Fh, y/Fw=0, 0.4) in this order.
 2. The intercooler according to claim 1, wherein the length ratio (x/Fh) is set to have a value of 0.1, and the protrusion ratio (y/Fw) is set to have a value in a range of 0.43-0.87.
 3. The intercooler according to claim 1, wherein the protrusion protrudes on a first side in the major direction by pressing the first wall from a second side opposite from the first side in the major direction.
 4. The intercooler according to claim 1, wherein the protrusion is long in the connecting direction and is narrow in the flowing direction of the intake air.
 5. The intercooler according to claim 4, wherein the protrusion has an ellipse shape that is long in the connecting direction and is narrow in the flowing direction of the intake air.
 6. The intercooler according to claim 1, wherein the protrusion is one of a plurality of protrusions arranged in the connecting direction.
 7. An intercooler comprising: a flat tube, intake air to be drawn into an engine passing through the flat tube and being cooled by external fluid, the flat tube having two major faces opposing with each other in a thickness direction; and an inner fin arranged inside of the flat tube, the inner fin having a wave-shaped cross-section constructed by alternately connecting a plurality of first walls and a plurality of second walls in a major direction approximately perpendicular to the thickness direction, the second wall being approximately parallel with the two major faces, the first wall connecting two of the second walls in a connecting direction corresponding to the thickness direction, wherein the first wall linearly extends in a flowing direction of the intake air that is approximately perpendicular to the connecting direction and the major direction, and the second wall has a protrusion protruding from an inner face of the second wall toward an opening side of the inner fin having the wave-shaped cross-section.
 8. The intercooler according to claim 7, wherein the protrusion is produced by pressing the second wall toward a center of the flat tube. 